The present invention is concerned with apparatus for installing a pipeline structure or pipeline structures at sea, particularly but not exclusively for installing, from a vessel, subsea pipeline accessories (such as pipeline end terminations (PLETs) and in-line tees (ILTs)) having bulky mudmats which support the accessory on the seabed. A method of installing pipeline accessories at sea is also provided.
When laying pipeline from a vessel onto the seabed it is often necessary to provide large mudmats for accessories such as PLETs or ILTs to rest upon while on the seabed. However, on many pipelaying vessels the laying equipment (tensioners, stinger etc.) being used to control deployment of the pipeline from the vessel results in the pipeline laying equipment having a restricted dimension, which limits the maximum size of structure which can pass through it. This therefore limits the size of the mudmat that can be deployed from the vessel without temporarily interrupting the pipelaying process. This is a particular problem for vessel equipped with S-lay pipelaying equipment. The attraction of S-lay methods is their high lay rate, but this is spoiled if the process is interrupted.
One known way of attempting to overcome this problem is the incorporation of folding components into the mud mat which, when folded, reduce its maximum dimension as it passes through the laying equipment. An example of such a folding arrangement is a mud mat provided with folding wings, which are folded inwards as it passes through the laying equipment restriction, and which are then folded out after it has passed the laying equipment restriction. However, such foldable arrangements suffer from a number of additional disadvantages. For example, there is an increase in the complexity and cost of such mudmats because they require additional moving parts compared to fixed dimension mudmats. Furthermore, the folding configurations on such mudmats are designed to reduce its dimension in order that it may pass through the laying equipment as it is deployed from the vessel. However, this folding of the mudmat often results in an increase in the storage area required at the stern of the vessel to store the mudmat prior to deployment. Such arrangements can also be unstable, as the folded mudmats having a relatively high centre of gravity.
Another alternative is to completely bypass the restriction in the laying equipment by passing the PLET or ILT over the side of the vessel away from the laying axis and then recovering it to the laying axis at the other side of the pipelaying equipment. This avoids the mudmat having to pass through the laying equipment altogether; however, handling of such a bulky accessory in this way can be a complex and time consuming operation.
According to the present invention, there is provided a method of deploying pipeline from a vessel having pipelaying equipment, the method comprising:—                installing an accessory support structure at an appropriate bedding location on the seabed;        attaching an accessory at a position along the pipeline which, when the pipeline is deployed from the vessel, will correspond with the pre-installed accessory support structure bedding location;        deploying the pipeline and attached accessory through the pipelaying equipment; and        guiding the attached accessory into engagement with the pre-installed accessory support structure on the seabed such that the pipeline is deployed substantially horizontally through the accessory support structure and the accessory is supported by the pre-installed accessory support structure at the bedding location.        
In this way accessory support structures that are too large to pass all the way through the pipelaying equipment, can be used, without significant interruption in the lay process.
The method of deploying pipeline from the vessel may be performed on a vessel having pipelaying equipment configured to lay pipe in an S-lay pipelaying operation.
The step of attaching accessories at positions along the pipeline may comprise the step of attaching in-line structures, such as an in line tee structures.
The step of attaching accessories at positions along the pipeline may comprise the step of attaching end terminations, such as pipeline end termination structures.
The step of installing the accessory support structure at an appropriate bedding location on the seabed may include the step of deploying a plurality of accessory support structures from the pipelaying vessel in a previous deployment pass of the vessel. Alternatively, the step of installing the accessory support structure at an appropriate bedding location on the seabed may include the step of deploying a plurality of accessory support structures from another vessel, which is not the pipelaying vessel, in a deployment pass of the other vessel prior to passage of the pipelaying vessel.
The step of attaching the accessory at a position along the pipe may involve the steps of:                measuring the installed position of an accessory support structure;        aligning the vessel relative to the position of an accessory support structure;        measuring the length of the pipeline deployed while laying the pipeline along a path leading to the position of the accessory support structure; and        attaching the accessory to the pipeline at a length along the pipeline that coincides with the expected longitudinal and lateral position of the installed accessory support structure on the seabed.        
The step of attaching the accessory at positions along the pipeline may involve clamping portions of the accessory around the pipeline.
The step of deploying the pipeline and attached accessory through the pipelaying equipment may comprise:                maintaining the accessory in a substantially upright configuration as it is deployed from the vessel towards the seabed.        
Maintaining the accessory in a substantially upright configuration may involve supporting the accessory on an angled roller arrangement as the accessory is deployed from the vessel. Maintaining the accessory in the substantially upright configuration may also involve attaching temporary buoyancy aids to the accessory.
The step of guiding each attached accessory into engagement with the associated pre-installed accessory support structure on the seabed may comprise guiding the pipeline and hence the attached accessory into a cradle provided on the accessory support structure.
The step of guiding each attached accessory into engagement with the associated pre-installed accessory support structures on the seabed may further comprise the step of laterally adjusting the position of each accessory by abutting portions of the accessory against a widened and angled mouth of the cradle, and progressing the accessory into the cradle such that the widened and angled mouth guides the accessory towards the centre of the accessory support structures.
The step of guiding each attached accessory into engagement with each associated accessory support structure on the seabed may further comprise the step of longitudinally adjusting the position of the accessory by sliding it on translation means provided between the accessory and the accessory support structure. The translation means may comprise skids provided on portions of the accessory and skids provided on portions of the accessory support structure.
According to the present invention, there is also provided a method of deploying pipeline from a vessel having pipelaying equipment, the method comprising:—                deploying a plurality of support structures at bedding locations on the seabed;        attaching a plurality of accessories at positions along the pipeline;        deploying the pipeline and attached accessories from the vessel; and        guiding each attached accessory towards an appropriate support structure in a bedding location on the seabed, wherein each said accessory is guided into supporting engagement with the removable support structure at the seabed.        
According to the present invention, there is also provided an accessory support structure for supporting a pipeline accessory on a seabed, said support structure comprising a cradle arrangement having a widened and angled mouth for guiding the accessory toward the centre of the accessory support structure as the accessory is lowered thereinto.
In a complementary aspect, the invention provides a support frame for a pipeline accessory for attachment to a pipeline being laid, wherein the support frame is adapted on attachment to the pipeline and deployment from a vessel having pipelaying equipment to be engaged with an accessory support structure as set out above.
In a related aspect, the invention provides apparatus for deploying a pipeline with a pipeline accessory on the seabed, comprising a pipeline accessory mounted in a support frame as set out above and a corresponding accessory support structure as set out above.
The pipelaying equipment may comprise S-lay pipelaying equipment.
The accessories may comprise in-line structures, such as an in line tee structures. The accessories may comprise end termination structures, such as pipeline end termination structures.
The accessory may be provided with a support frame having a relatively wide upper support platform and a relatively narrow lower support platform in order to define an accessory having a tapered support platform profile when viewed along the accessory's longitudinal axis.
The edges of the support platforms may be provided with translation means. The translation means may comprise skid pads.
The accessory may be provided with clamping portions for attaching the accessory to the pipeline being laid. A lower clamping portion may be provided in order to securely clamp the accessory to the pipeline being laid. An upper clamping portion may also be provided in order to securely clamp the accessory to a manifold pipe. The clamping portions may comprise a tubular clamp arrangement. The support frame may have two support frame portions, each with an upper clamping portion and a lower clamping portion.
A resilient collar may be provided on the upper and/or lower clamping portion to improve the grip provided by the clamping portions on the pipeline being laid. The resilient collar may comprise neoprene.
The accessory support structure may have a cradle arrangement comprising a widened and angled mouth for guiding the accessory toward the centre of the accessory support structure as the accessory is lowered thereinto. The widened and angled mouth may have skids adapted to assist progression of the accessory thereinto.
The accessory support structure also preferably comprises a substantially planar mudmat for resting the weight of the accessory support structure on the seabed.
At least a longitudinal support rail may be provided on the planar mud mat to support a portion of the accessory thereon. The height of the, or each, longitudinal support rail is configured to ensure that, when the accessory is attached to the pipeline, sufficient clearance remains between the bottom of the pipeline and the upper mudmat surface. It may be preferable to provide two parallel longitudinal support rails on the mudmat in order to lower the pipe as much as possible and to minimise high points in the pipeline.
The cradle arrangement may be provided as left and right frame members comprising a series of upright frames linked by at least a longitudinal sliding rail spanning between the series of upright frames to allow the accessory to slide along the sliding rails once supported by the cradle arrangement.
It may be preferable to provide an upper longitudinal sliding rail and a lower longitudinal sliding rail on either side of the cradle arrangement. The upper and lower longitudinal sliding rails on one side of the cradle may be laterally spaced apart from those on the other side of the cradle by a distance which coincides with the tapered profile of the accessory frame.
The upper longitudinal sliding rail acts to prevent rotation of the accessory, which could otherwise occur due to forces applied by e.g. a jumper. This is desirable because the lever arm between the two lower longitudinal sliding rails may not be sufficient to prevent rotation (since they must be relatively close together in order to pass through the stinger). The upper longitudinal sliding rails combined with the lower longitudinal sliding rails therefore provide the required support against rotation of the accessory.
Translation means may be provided between the tapered support platform and the accessory support structure in order to assist lateral movement of the accessory with respect to the accessory support structure in service and or during installation. The translation means may comprise skids provided on the tapered support platform of the accessory and a skidding surface provided on the longitudinal sliding rails of the cradle arrangement.
Deployment control means may be provided on the vessel to control deployment of the accessory and pipe from the vessel. The deployment control means may comprise an angled roller arrangement to maintain the attached accessory in an upright configuration as it is being deployed from the vessel. The angled roller arrangement may be provided on a stinger member provided at the rear of the vessel.
In a further aspect, the invention provides apparatus for deploying a pipeline with an installed pipeline accessory from a pipelaying vessel, the apparatus comprising deployment control means provided on the vessel to control deployment of the accessory and pipe from the vessel such that the pipeline accessory is maintained in a substantially upright configuration.
Advantageously, the deployment control means may comprise an angled roller arrangement to maintain the attached accessory in a substantially upright configuration as it is being deployed from the vessel. The apparatus may further comprise buoyancy aids installed temporarily on the pipeline while lowered from the pipelaying vessel to the seabed to maintain the attached accessory in a substantially upright configuration between deployment from the vessel and laying the pipeline on the seabed.